Optical fibers are waveguides that can transmit light, with minimal scattering and attenuation, between two locations. Optical fibers, also sometimes called fiber optics, are well known and used, for example, for illumination, communications, information transfer, and sensors. Optical fibers are typically flexible and very thin, and have a transparent core surrounded one or more transparent cladding layers. The core and cladding layers are made of vitreous material, such as high quality glass (made from, e.g., silica, fluoride, phosphates, etc.). Typically, the core material has a refractive index which is greater than the refractive index of the material in the surrounding cladding layer or layers. These conditions enable total internal reflection of light signals passing through the fiber, resulting in an efficient waveguide.
Optical fibers are generally manufactured by drawing the fiber from a heated preform using a fiber drawing tower. Such towers are typically vertically oriented and have a guide to hold and guide a preform, end first, into the top of the tower, as well as a high temperature furnace to heat the preform in a controlled manner, and an apparatus to apply controlled tension to the leading end of the preform, whereby a fiber of molten material forms. The fiber is typically cooled and solidified as it is drawn from the preform to provide a fine continuous optical fiber.
Optical components, either in the form of intermediate products (preforms or simple solid cylinders) for an optical fiber or also directly in the form of the end product itself in the form of the optical fiber, are produced by collapsing and elongating an arrangement including a core rod and a cladding tube surrounding the core rod. In some cases, multiple cladding tubes may be used. This process is typically referred to as rod-in-tube (RIT) or rod-in-cylinder (RIC).
In this method the core rod is positioned within the cladding tube in a vertical arrangement. In some cases, the core rod may be supported at its bottom by a support rod inserted into the bottom of the cladding tube. In other cases, the core rod may be supported by a holding ring or disk positioned in a constricted bottom portion of the cladding tube. The holding ring or disk has an outer diameter smaller than the inner diameter of the cladding tube, but larger than the inner diameter of the constricted portion, so that the holding ring comes to rest from above on the area of the constricted portion. The gap between the outer diameter of the core rod and the inner diameter of the cladding tube is sealed at one end of the core rod and a vacuum is applied to the gap from the other end of the core rod. The cladding tube and core rod are then heated while maintaining the vacuum, resulting in the cladding tube collapsing around the core rod. In other processes, the cladding tube and core rod may be heated prior to applying the vacuum to the gap between the core rod and the cladding tube.
One drawback of the above method is that the core rod is pulled down by its own weight while heating the outer cladding tube, resulting in deformation leading to an inconsistent outer diameter of the core rod, the core rod slipping out of its intended position within the cladding tube, or both. An inconsistent outer diameter of the core rod or the core rod not being in the correct position can lead to variations in the “b/a ratio” of the glass component (i.e., the ratio of the cladding tube diameter to the core rod diameter for a given cross section of the glass component). In some applications such as optical fibers, even small deviations from the desired b/a ratio are unacceptable. In extreme cases, weight-related deformation of the core rod can even lead to the core rod breaking. Core rod deformation is increasingly a problem as the length of the core rod increases. Although the application of a negative pressure to the gap between the outer diameter of the core rod and the inner diameter of the cladding tube can counteract the gravitational forces acting on the core rod, there arises a maximum pressure difference between the gap and the outside of the cladding tube, particularly when atmospheric pressure is applied externally.
Typical solutions to preventing weight-related deformation include using two core rod segments, where the bottom segment is supported by a support rod or holding ring as described above and the top segment is supported at a point above the bottom segment. For example, in U.S. Pat. No. 8,161,772, the cladding tube includes a necked portion of reduced inner diameter. The top core rod segment is then supported by the necked portion, either directly by the necked portion or by a spacer disk supported by the necked portion. However, such methods are generally undesirable because they require core rods of different diameters; hot working, welding, or machining of the core rods or cladding tubes; or both. Requiring core rods of specific diameters reduces the possible configurations the methods can produce, while hot working, welding, or machining can increase costs and causes stresses on the components that can reduce quality or reliability.